Orthographic Projection (First Angle and Third Angle)

TL;DR

Orthographic projection is how we show 3D objects as a set of 2D views, typically front, top, and side. There are two main methods: First Angle and Third Angle, which differ in where you imagine the viewer and the projection plane. Understanding these methods is crucial for accurately communicating design intentions in technical drawings.

1. The Mental Model

Imagine an object floating in space. To draw it on paper, you look at it straight on from different directions (front, top, side) and capture what you see, as if you're flattening it onto a transparent screen.

2. The Core Material

Orthographic projection is a fundamental technique in technical drawing that allows you to represent a 3D object using multiple 2D views. Each view shows the object as if you're looking directly at one of its faces, perpendicular to that face. These views are then arranged in a specific way on a drawing sheet.

The goal is to provide enough views so that someone looking at the drawing can completely understand the object's shape, size, and features without ambiguity. Typically, three views are used:
* Front View (or Elevation): Shows the most descriptive face of the object.
* Top View (or Plan): Shows the object looking down from above.
* Side View (or End View): Shows the object from either the left or right side.

The relationship between these views is critical; features in one view must align precisely with features in another. Hidden lines (dashed lines) are used to indicate edges or features that aren't visible from that particular viewpoint but are part of the object.

There are two main standards for arranging these views:

First Angle Projection

Photo by Ann Hannes on Pexels

In First Angle Projection, you imagine the object is placed in front of the projection planes. The object is between you and the projection plane. When you draw the view, it's placed on the opposite side of the object from where you're looking.

• If you look at the front of the object, you draw the front view below the actual front face, on the paper.
• If you look at the top of the object, you draw the top view below the front view.
• If you look at the left side of the object, you draw the left side view to the right of the front view.

Think of it like this: you're looking through the object onto the paper behind it. This method is predominantly used in Europe, Asia, and other parts of the world.

Third Angle Projection

Photo by Aa Dil on Pexels

In Third Angle Projection, you imagine the projection planes are placed between you and the object. You're looking through the plane at the object, and you draw what you see directly onto that plane.

• If you look at the front of the object, you draw the front view directly where you'd expect it.
• If you look at the top of the object, you draw the top view above the front view.
• If you look at the left side of the object, you draw the left side view to the left of the front view.

This method is commonly used in the United States, Canada, and Australia. It's often considered more intuitive because the views are arranged as if you're rotating the object into position to see each face.

Projection Symbols

Photo by Ann Hannes on Pexels

To avoid confusion, every orthographic drawing includes a symbol to indicate whether it's First Angle or Third Angle Projection.

graph TD
    A["Orthographic Projection"] --> B["First Angle Projection"]
    A --> C["Third Angle Projection"]

    B --> B1["Object is BETWEEN Viewer & Plane"]
    B --> B2["Views Placed OPPOSITE from Viewpoint"]
    B --> B3["Example: Top View is BELOW Front View"]
    B --> B4["Common in Europe/Asia"]

    C --> C1["Plane is BETWEEN Viewer & Object"]
    C --> C2["Views Placed IN LINE with Viewpoint"]
    C --> C3["Example: Top View is ABOVE Front View"]
    C --> C4["Common in US/Canada"]

    B1 & C1 --> D["Key Difference: Arrangement of Views"]

3. Worked Example

Let's consider a simple L-shaped block. We'll draw its front, top, and right-side views using both projection methods.

Imagine the L-block stands upright, with the longer vertical leg on the left, and the shorter horizontal leg extending to the right at the base. We'll define the front view as looking at the 'L' shape directly.

Given Object: An L-shaped block.
* Front View (looking at the 'L'):
* First Angle: The front view (the 'L') would be drawn in the main position.
* Third Angle: The front view (the 'L') would be drawn in the main position. (No difference here for the primary view)

• Top View (looking down from above):

  • First Angle: When you look down from the top, you'd see two rectangles. Since the object is between you and the plane, you place this view below the front view.
  • Third Angle: When you look down from the top, you'd see two rectangles. Since the plane is between you and the object, you place this view above the front view.

• Right Side View (looking from the right side):

  • First Angle: Looking from the right, you'd see two stacked rectangles. Since the object is between you and the plane, you place this view to the left of the front view. (You're looking from the right, but projecting to the left).
  • Third Angle: Looking from the right, you'd see two stacked rectangles. Since the plane is between you and the object, you place this view to the right of the front view.

The key is that the views must align. For instance, the width of the front view must match the width of the top view, and the height of the front view must match the height of the side view.

4. Key Takeaways

• Orthographic projection represents 3D objects with multiple 2D views.
• The standard views are typically front, top, and a side view (left or right).
• First Angle Projection places the object between the viewer and the projection plane.
• Third Angle Projection places the projection plane between the viewer and the object.
• First Angle views are arranged opposite the direction of viewing (e.g., top view below front).
• Third Angle views are arranged in the same direction as viewing (e.g., top view above front).
• A specific projection symbol must always be included on the drawing to indicate the method used.

Common Mistakes to Avoid:
- Mixing the methods: Never combine First Angle and Third Angle rules on the same drawing. Pick one and stick to it consistently.
- Misaligning views: All features must align correctly between adjacent views; a misalignment indicates an error in projection.
- Forgetting hidden lines: Crucial for showing features not directly visible in a view; omitting them leads to an incomplete representation.
- Incorrectly placing the projection symbol: The symbol is standard and must be drawn accurately and clearly on the drawing sheet.

5. Now Try It

Sketch a simple object like a rectangular block with a single hole drilled through its top face.
1. Draw the front, top, and right-side views using First Angle Projection.
2. Then, on a separate sheet, draw the same three views for the same object using Third Angle Projection.
3. Pay close attention to the alignment of features and the placement of the views relative to each other for both methods. Include the correct projection symbol for each drawing.

Success means you'll have two clear sets of drawings, both representing the same object, but with the views arranged differently according to the chosen projection method, and each drawing clearly labelled with its projection symbol.